Hal Murray said: >They make 74xU04 for many values of x. The U is for Unbuffered. They have lower gain in the linear region. I thought they were intended to be used for things like this, but I don't understand that area. Can anybody give me a quick lesson or point me at a good URL?<
I always thought the unbuffered "U" versions were preferred for ring oscillators mostly to save power - you don't want the high-drive output stages to be cooking away in linear mode if not needed. The propagation delay can also be less since the U ones have only one stage instead of three (the building block is the totem-pole inverter stage), but they can't drive very much load anyway. I think that most MSI and LSI parts that have built-in ring/crystal oscillator sections use the U topology, but I don't think there's anything special about it - it's the simplest thing that works.
I've made quite a few CD4000 and 74HC oscillators, and never worried too much about U versions or not, except for battery-run items where power is critical (or you can run the oscillator at lower voltage). Often they are made from inverting gates that are part of a shared package, where you wouldn't want puny drive capability in the other gates anyway. They are relative power hogs though, whenever linear biasing is needed. Except in the 4000 series, I don't know if U versions are available in anything but the '04 hex inverter, but I suppose it's possible. I think the Schmitt-trigger types like HC14 are necessarily buffered, so have three stages, since you need a non-inverted version of the signal for the positive feedback to the input.
I've never tried making one in 74AC - I don't know if it's even possible to bias one up that way without it burning up. I'm working on some related circuits now, so maybe I'll set up an experiment to see how much current it would take for one inverter - I've often wondered about this.
I read about this years ago in various CMOS application notes, so I may be missing some key points - there should be plenty of info online. The older generation (when CMOS was fairly new) info may provide more detail about the guts than that related to the newer, higher performance families.
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